Process for producing a steel strip comprising a relatively high strength dual phase steel

ABSTRACT

A relatively high strength dual phase steel for a cold-rolled or hot-rolled steel strip with excellent forming properties, in particular for lightweight vehicle construction, contains the elements (contents in mass-%): 0.1 to &lt;0.16 of C, 0.02 to &lt;0.05 of Al, 0.40 to &lt;0.60 of Si, 1.5 to &lt;2.0 of Mn, &lt;0.020 of P, &lt;0.003 of S, &lt;0.01 of N, 0.01 of Nb, 0.02 of V, remainder iron including common incidental steel elements with optional addition of Ti. The demanded dual phase microstructure is produced during continuous annealing, wherein the cold-rolled or hot-rolled steel strip is heated in the continuous annealing furnace in a one-step process to a temperature in the range of 820 to 1000° C., preferably 840 to 1000° C., and the annealed steel strip is then cooled down from the annealing temperature with a rate of cooling between 15 and 30° C./s.

The invention relates to a process for producing a cold-rolled orhot-rolled steel strip of a relatively high strength dual phase steelwith excellent forming properties, in particular for lightweight vehicleconstruction according to the preamble of claim 1.

The hotly contested automobile market forces the manufacturer i.a. tolook continuously for solutions to lower the fleet consumption whilemaintaining a highest possible comfort and greatest possible occupantprotection. A crucial role plays hereby weight saving of all vehiclecomponents, on the one hand, but also a beneficial behavior of theindividual components when exposed to high static and dynamic stressduring operation and in the event of a crash, on the other hand.Suppliers attempt to take this requirement into account in such a waythat the wall thickness can be reduced through use of high strength andsuper high strength steels while at the same time improving thecomponent behavior during its manufacture thereof and at operation. Suchsteels have to meet therefore comparably high standards with respect tostrength, stretching capacity, toughness, energy consumption andworkability, for example by cold forming, welding and/or surfacetreatment.

So-called dual phase steels find increasingly application in this areaas a result of their excellent formability and high strength values atthe same time. Dual phase steels have hereby mainly ferritic-martensiticstructure.

Considered in this context are steel strips of dual phase steel whichare cold-rolled as well as hot-rolled.

For economic reasons, cold-rolled steel strips are normally subjected torecrystallization annealing by way of a continuous annealing processinto a metal sheet that is easy to shape.

The furnace parameters (run-through speed, annealing temperature, rateof cooling) are adjusted in dependence on the alloy composition andstrip thickness in accordance with the demanded microstructure andmechanical-technological properties.

The dual phase microstructure is adjusted by heating the cold bath inthe continuous annealing furnace to such a temperature that the requiredferritic-martensitic microstructure is formed during cooling.

When high corrosion standards demand that the surfaces of hot or coldstrips should be galvanized through hot dipping, the annealing treatmentis normally carried out in a continuous annealing furnace upstream ofthe galvanizing bath.

Also in the case of the hot strip, the required dual phasemicrostructure is occasionally adjusted depending on the alloyingconcept only during annealing treatment in the continuous furnace inorder to be able to realize the demanded mechanical properties on thebasis of an austenitic microstructure which is as homogenous aspossible.

The alloying concepts for dual phase steels known for example from thedocuments EP 0 152 665 B1, EP 0691 415 B1, and EP 0510 718 B1, for usein continuous annealing of hot-rolled or cold-rolled steel strips areproblematic because of the presence of only a narrow process window forthe annealing parameters to ensure-uniform mechanical properties overthe length of the strip.

In order for the steels to attain a transformation inertia that issufficient for realizing the demanded dual phase microstructure, whenthe cold strip undergoes recrystallizing annealing, the known steelshave respective contents, e.g. of Cr, Mo, Nb, or B. In particular thecostly elements Cr and Mo have an adverse impact on the manufacturingcosts of the dual phase steel.

A narrow process window is to be understood in this context as a need toadjust the run-through speed in dependence on thickness of the strip tobe annealed in order to attain a homogenous temperature distribution inthe strip and the demanded dual phase microstructure and themechanical-technological properties during cooling.

When the process windows are wide, the demanded strip properties can berealized even when the strips to be annealed have different thicknesswhile the furnace parameters remain the same.

During manufacture, it is oftentimes required to anneal successivestrips of different thickness, e.g. 1.5 and 2.0 mm, depending onspecification.

A homogenous temperature distribution is difficult to realize inparticular in the transition zone from one strip to another, whendifferent thicknesses are involved, and lead in the event of alloycompositions with too small process window to a situation in which theadvance of the thinner strip through the furnace is too slow, causing alower productivity, or the advance of the thicker strip through theannealing furnace is too fast, posing the risk of failure to realize ahomogenous temperature distribution and thus the demandedmechanical-technological properties. As a result, increasing waste andeven customer complaints are encountered.

The problem of an excessively narrow process window is especiallyegregious during annealing treatment when load-optimized components ofhot or cold strip should be produced which have varying sheetthicknesses in length and, optionally, across the width of the strip,i.e. have been rolled flexibly. A process for producing a steel strip ofvarying thickness over the strip length is described, e.g., in DE 100 37867 A1.

When applying the known alloying concepts for dual phase steels, thepresence of the narrow process window renders the realization of uniformmechanical properties difficult to achieve over the entire strip lengthof the respective strip when a continuous annealing of strips of varyingthicknesses is already involved.

When the process window is too small, the regions of smaller sheetthickness in flexibly rolled hot or cold strips of steel of knowncompositions have strengths that are too low as a result of thesubstantial proportion of ferrite in view of the transformationprocesses during cooling, or the regions of greater sheet thicknessreach values that are too high as a result of the substantial proportionof martensite. Homogenous mechanical-technological properties over thestrip length or across the strip width are virtually impossible toattain, when using the known alloying concepts during continuousannealing.

The invention is therefore based on the object to provide a differentmore cost-efficient alloying concept for a relatively high-strengthsteel with dual phase microstructure that allows a broadening of theprocess window for continuous annealing of hot or cold strips in such away that in addition to strips of varying thickness also steel strips ofvarying thickness over the strip length and, optionally, across thestrip width can be produced having mechanical-technological propertieswhich are as homogenous as possible.

According to the teaching of the invention, this object is solved by asteel having the following contents in mass-%:

C 0.1 to ≦0.16 Al 0.02 to ≦0.05 Si 0.40 to ≦0.60 Mn 1.5 to ≦2.0 P≦0.020S≦0.003 N≦0.01 Nb=0.01 V=0.02

remainder iron including common incidental steel elements with optionaladdition of Ti, wherein the demanded dual phase microstructure isproduced during continuous annealing, and wherein the cold-rolled orhot-rolled steel strip is heated in the continuous annealing furnace ina one-step process to a temperature in the range of 820 to 1000° C.,preferably 840 to 1000° C., and the annealed steel strip is then cooleddown from the annealing temperature with a rate of cooling between 15and 30° C./s.

The relatively high strength dual phase steel in accordance with theinvention for the lightweight vehicle construction is characterized inthat the targeted addition of V and Nb while omitting the cost-intensivealloying elements Mo or CR results in a transformation inertia which ishigh enough to enable with very high process reliability an adjustmentof the demanded dual phase microstructure with homogenousmechanical-technological properties during continuous annealing from acompletely austenitic matrix even when strips are involved having athickness which varies over the strip length or across the strip width.

Comprehensive laboratory experiments have surprisingly found that atargeted addition of V in combination with Nb provides a dual phasesteel which allows a significantly broader process window duringcontinuous annealing. Same microstructure formations andmechanical-technological properties of the strips can be realized evenwhen strips of different thickness or strips with varying thickness areannealed at otherwise constant furnace parameters.

The steel according to the invention offers the benefit of asignificantly greater process window compared to known steels. As aresult, process reliability is enhanced during continuous annealing orhot dip galvanizing of cold and hot strips with dual phasemicrostructure. Thus, homogenous mechanical-technological properties inthe strip can be assured in hot-galvanized as well as continuouslyannealed hot or cold strips. This applies for continuous annealing ofsuccessive strips with different strip thickness and in particular forstrips with varying sheet thickness over the strip length and/or stripwidth.

When in accordance with the invention relatively high-strength hot orcold strips of varying sheet thicknesses are produced by a continuousannealing process, load-optimized components can be advantageouslymanufactured from this material through shaping.

In accordance with the invention, a dual phase steel is involved havingapprox. 20% martensite embedded in the form of islands in the strengthclass of about 800 MPa. in particular for hot dip galvanizing as well asfor the application in a continuous annealing facility.

As a consequence of the optional addition of Ti in contents of ≦0.01%,the fine-grained configuration of the microstructure and themechanical-technological properties can be adjusted in accordance withthe invention via the formation of nitrides or carbonitrides independence on the N-content of the steel.

The field of application of the steel for rolling with flexible stripthicknesses in longitude and transverse directions with respect to therolling direction is opened up as a result of its insensitivity againstprocess fluctuations during heat treatment.

This insensitivity is effectuated by the use of Nb and in particular Vwhich cause a transformation-inert or transformation-free zone duringcooling.

In order to attain a respective effect, the steel has in accordance withthe invention a V content of at least 0.02% and a Nb content of at least0.01%. Nb acts hereby as grain refining element, with the extent of theNb addition being suited to the actual C and N contents of the steel.

The addition of V is also adjusted in accordance with the invention tothe contents of C and N, with the extent of the addition being suitedhowever in such a way that enough V is kept in solution in order torealize a sufficient transformation inertia. When desiring a behaviorthat is as transformation-inert as possible and thus to realize abroadest possible process window during continuous annealing, the Vcontent amounts to at least 0.06 to 0.10% and the Nb content to morethan 0.02 to 0.05%. Further increase of the contents of V and Nb doesnot provide any further benefits as far as a further retardedtransformation of the steel is concerned and thus for the broadness ofthe process window during continuous annealing.

In order to attain a substantially homogenous starting microstructurefor adjustment of the dual phase microstructure, the annealed strip isfirst heated to a temperature that causes a completely austeniticmicrostructure. The annealing temperatures range hereby for the steelaccording to the invention between approx. 820 and approx. 1000° C.,depending on the concrete alloy composition.

Performed tests have shown that this steel has a zone which does notundergo a reverse transformation of austenite into ferrite, bainite, ormartensite despite temperatures of less than 800° C. Important is herebyin particular the temperature range of about 450° C. because thegalvanizing bath temperature is hereby at a level for hot dipgalvanizing.

The adjusted content of ferrite and (residue) austenite during coolingis maintained until after the process step “galvanizing”. The stillpresent proportion of austenite is then fully transformed intomartensite during continued cooling. The galvanizing parameters may varyover a wide range. The galvanizing speeds range between 60 and 120 m/mindepending on the sheet thickness. The rate of cooling before and afterthe galvanizing bath ranges at fairly low 10 to 30° C./sec.

The produced material may be processed as cold bath as well as also hotbath, in dressed and undressed but also heat-treated state (intermediateannealing) via a hot dip galvanizing line or a pure continuous annealingfacility.

At the same time, there is the possibility to vary the coolingconditions in a targeted manner before the galvanizing bath in order toincrease or decrease the proportion of ferrite. As a result, it ispossible to, e.g., produce partly martensitic steels (PM).

1.-8. (canceled)
 9. A process for producing a cold-rolled or hot-rolledsteel strip of a high strength dual phase steel having excellent formingproperty, comprising the steps of: selecting a cold-rolled or hot-rolledsteel strip with a composition comprising, in mass-%, 0.1 to ≦0.16 of C,0.02 to ≦0.05 of Al 0.40 to ≦0.60 of Si 1.5 to ≦2.0 of Mn ≦0.020 of P≦0.003 of S ≦0.01 of N 0.01 of Nb 0.02 of V, remainder iron includingcommon incidental steel elements; heating the steel strip in acontinuous annealing furnace in a one-step process to an annealingtemperature in the range of 820 to 1000° C.; and cooling the annealedsteel strip from the annealing temperature with a rate of coolingbetween 15 and 30° C./s.
 10. The process of claim 9, further comprisingthe step of adding Ti.
 11. The process of claim 9, wherein the steelstrip is heated to a temperature of 840 to 1000° C.
 12. The process ofclaim 9, wherein the content of V is 0.06%.
 13. The process of claim 9,wherein the content of V is 0.08%.
 14. The process of claim 9, whereinthe content of Nb is 0.02%.
 15. The process of claim 9, wherein thecontent of Nb is 0.04%.
 16. The process of claim 10, wherein the contentof Ti is ≦0.01%.
 17. The process of claim 9, further comprising the stepof refining the steel strip by hot dipping.
 18. The process of claim 17,further comprising the step of dressing the steel strip.
 19. The processof claim 9, wherein the cold-rolled or hot-rolled steel strip of dualphase steel is used for lightweight vehicle construction.